1. Technical Field
This invention relates to lens testing systems and more particularly to a system for testing aspheric surfaces utilizing a phase modulated Ronchi interference pattern.
2. Discussion
Lenses made with aspheric surfaces are frequently employed to correct for spherical aberrations. In an aspheric lens the curve of the lens is not defined by a circle. Grinding aspheric surfaces is a difficult and tedious process which requires precise manufacturing techniques.
Equally important to the quality of the resulting lens is the accuracy of the technique used to measure the shape of the lens surface at various stages in the manufacturing process. Various methods have been employed to measure lens surface contours. One system has been developed to perform such measurements simultaneous with the manufacturing process. For example, see U.S. Pat. No. 4,561,333 issued to Kohno, et. at. This patent discloses a technique for performing the Hartmann test during diamond turning of a metal mirror.
The Ronchi test is another well known technique for performing qualitative tests on lens surfaces. A Ronchi grating is an optical device consisting of ruled lines of a certain frequency. In the Ronchi test, this fringe pattern is projected onto the lens surface under test, and the fringes reflected off the lens will then be distorted in a predictable way depending on the shape of the surface of the lens under test. Deviation from the predicted reflected fringe shape will represent geometric aberrations or defects in the lens surface. Upon detecting such aberrations, the lens may then be corrected by further manufacturing processes.
Of course, the quality of the resulting lens or mirror surface will depend on the accuracy of the measuring technique. The Ronchi test as previously employed has had limited accuracy in testing aspheric surfaces. Other methods have been developed, such as the use of computer generated holograms, shearing interferometers, and the deformed grating method. In all of these methods, the accuracy is limited. U.S. Pat. No. 4,212,073 issued to N. Balasubramanian, describes an improved Ronchi test. This technique however, requires sophisticated analysis of the detected signal.
A quantitative Ronchi test utilizing a synchronous phase detection technique, is described in T. Yatagai, "Fringe scanning Ronchi test for aspherical surfaces", 23 Applied Optics 3676 (1984). This technique employs a system which utilizes a moveable Ronchi grating for testing light passing through a refractive lens. However, this technique is not suitable for testing a mirror surface or for testing a single surface of a refractive lens in a reflecting mode. Further, in this and other prior methods of Ronchi testing, the selection of the spatial frequency of the ruling resulted in a compromise in measurement precision. This is because the use of a too high frequency ruling will cause diffraction, causing the wave fronts to interfere and reducing the precision of the measurement; a too low frequency ruling introduces geometric measurement errors.
Thus, it would be desirable to provide a way to accurately measure a single aspheric surface in the reflective mode which does not require elaborate instrumentation. It would further be desirable to provide an accurate Ronchi test which can be performed in a manufacturing setting without the need for a laboratory environment.